powerpc: Update kernel VSID range

This patch change the kernel VSID range so that we limit VSID_BITS to 37.
This enables us to support 64TB with 65 bit VA (37+28). Without this patch
we have boot hangs on platforms that only support 65 bit VA.

With this patch we now have proto vsid generated as below:

We first generate a 37-bit "proto-VSID". Proto-VSIDs are generated
from mmu context id and effective segment id of the address.

For user processes max context id is limited to ((1ul << 19) - 5)
for kernel space, we use the top 4 context ids to map address as below
0x7fffc -  [ 0xc000000000000000 - 0xc0003fffffffffff ]
0x7fffd -  [ 0xd000000000000000 - 0xd0003fffffffffff ]
0x7fffe -  [ 0xe000000000000000 - 0xe0003fffffffffff ]
0x7ffff -  [ 0xf000000000000000 - 0xf0003fffffffffff ]

Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Tested-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.8]